USB ports are found on a variety of electronic devices including desktop computers, automobile dashboard consoles and other host devices as well as battery-powered portable devices such as laptop computers, tablets, mobile phones, e-readers, MP3 players, etc. USB ports are accessible via standardized USB cable connections to provide serial communications between devices, as well as electrical power transfer for charging and operating battery-powered peripheral devices. Moreover, dedicated charging devices are available having multiple USB ports for charging various portable devices, which may include circuitry for fast charging certain peripheral devices. USB compatible systems typically include interface chips mounted to an internal circuit board to interface USB data and power connections to host system circuitry such as power circuits and host processors. Electronic devices are typically tested for compliance with ESD test standards to confirm the ability to safely conduct transient ESD currents. Certain Human Body Model (HBM) tests involve conduction of a current of approximately 1.3 A at 2 kV, whereas more rigorous standards require conduction of higher transient currents. For instance, the 8 kV IEC61000-4-2 contact discharge and ISO 10605 standards verify protection against transient currents at or near 30 A.
Many circuit boards having USB connectors and interface circuitry include dedicated ESD protection integrated circuits (ICs) or discrete ESD protection circuit components mounted to the circuit board for compliance with the applicable standards. For USB data lines, however, many conventional ESD protection circuits are not feasible or desired. For instance, high-speed data operation on positive and negative USB data lines (DP and DM, also indicated as D+ and D−) requires minimal resistance and stray capacitance on the data lines, thus rendering resistive or capacitive clamping type ESD protection circuitry unsuitable for USB data line protection. Moreover, external clamping diodes may add undesirable stray capacitance to the data lines.
Many different circuit solutions have been used for protecting circuitry against ESD events in general, such as dedicated NMOS-based or other active ESD circuits, but these solutions significantly increase the die area and cost of an integrated circuit solution. Furthermore, dedicated ESD ICs and/or other board level circuitry occupies valuable board space, and can unduly increase the impedance with respect to USB data lines. For example, external IEC protection components (i.e. off-chip IEC diodes) for the USB DP and DM lines are not preferred due to added cost and parasitic capacitance on the system board. The impacts of implementing on-chip system-level ESD structures include significantly growing the IC die area and increasing of total IC cost. Conventional solutions therefore often include external protection diodes or clamping circuits placed on system board, where the ESD current is discharged through the external diode to ground. However, the clamping voltage is usually high for external ESD protection structures and providing sufficient ESD protection for advanced ESD standards is therefore difficult or impossible without adding undesirable series resistance and stray capacitance on the USB data pins, and the use of such external protection diodes increases the total system cost and size. Moreover, provision of on-chip ESD structures requires significant increase in the IC die space.